doc: update information compilation with clang

doc/installation.rst

@@ -9,10 +9,13 @@ Knot DNS Installation
 Required build environment
 ==========================
 
-GCC at least 4.1 is strictly required for atomic built-ins, but 4.2 or
-newer is recommended.  Another requirement is ``_GNU_SOURCE`` support,
-otherwise it adapts to the compiler available features.  LLVM clang
-works, but it is not officially supported.
+GCC at least 4.1 is strictly required for atomic built-ins, but the latest
+available version is recommended. Another requirement is ``_GNU_SOURCE``
+support, otherwise it adapts to the compiler available features.
+
+LLVM clang compiler can be used as well. However, the compilation with
+enabled optimizations will take a long time, unless the ``--disable-fastparser``
+configure option is given.
 
 Knot DNS build system relies on these standard tools:
 

doc/requirements.rst

@@ -43,4 +43,4 @@ Supported operating system
 Knot DNS itself is written in a portable way, but it depends on
 several libraries. Namely userspace-rcu, which could be a constraint
 when it comes to the operating system support. Knot DNS can be compiled
-and run on most of UNIX-like systems, such as Linux, \*BSD and Mac OS X.
+and run on most of UNIX-like systems, such as Linux, \*BSD, and OS X.